In cell-based transmission systems congestion can occur because of such factors as unpredictable statistical fluctuation of traffic flows and fault conditions within the network. Traffic control in such systems refers to the set of actions taken by the network to avoid congested conditions. In an ATM system, traffic parameters describe traffic characteristics of an ATM connection. A traffic parameter is a specification of a particular traffic aspect. It may be quantitative or qualitative. Traffic parameters may for example describe Peak Cell Rate. Sustainable Cell Rate, Burst Tolerance, and/or source type (e.g., telephone, video phone). The list of traffic parameters that can be used to capture the traffic characteristics of an ATM connection is referred to as the ATM Traffic Descriptor. There are various subsets of An ATM Traffic Descriptor such as a Source Traffic Descriptor and a Connection Traffic Descriptor.
The Connection Traffic Descriptor specifies the traffic characteristics of the ATM Connection at a public or private User Network Interface (UNI). It is made up of the set of parameters in the Source Traffic Descriptor, the Cell Delay Variation (CDV) Tolerance and the Conformance Definition that is used to unambiguously specify the conforming cells of the ATM connection. The characteristics of an ATM Connection at a private or public UNI, which are negotiated, are referred to as the Traffic Contract. The conformance definition used to specify the conforming cells of an ATM Connection at the UNI are based on a generic cell rate algorithm. There are two Cell Rate Algorithms that are in general use for determining whether a given cell conforms with the Traffic Contract, namely, the Virtual Scheduling Algorithm and a continuous-state Leaky Bucket Algorithm. However, in making these algorithms realizable in hardware in which finite word lengths must be used to represent both the arrival times and the other parameters, false identifications of non-conforming cells occur. In addition, with very slow rate connections, there is a further problem of reliability in identifying non-compliant cells.
Accordingly, it is an object of the invention to provide a method and apparatus for carrying out a cell rate time of arrival algorithm which avoids false identifications of non-conforming cells.